Electrode structure and method of fabricating the same

ABSTRACT

A reliable electrode structure capable of ensuring a sufficient width for a second conductive layer is provided. The electrode structure comprises a first conductive layer having first side walls and containing at least either polycrystalline silicon or amorphous silicon, a second conductive layer, formed on the first conductive layer, having second side walls and containing a metal and silicon, and side wall oxide films formed to be in contact with the first side walls and the second side walls. The first conductive layer and the second conductive layer contain nitrogen in the vicinity of the first and second side walls. The nitrogen concentration in the second side walls is larger than the nitrogen concentration in the first side walls.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrode structure and a method offabricating the same, and more particularly, it relates to an electrodestructure employed as a gate electrode of a semiconductor device and amethod of fabricating the same.

2. Description of the Prior Art

A structure formed by a first layer of polysilicon, for example, and asecond layer, located on the first layer, of metal silicide is recentlyemployed for a gate electrode forming a semiconductor device, in orderto reduce electric resistance. When such a gate electrode is applied toa DRAM (dynamic random access memory), for example, the gate electrodeis first formed by etching, and side walls of the gate electrode and theupper surface of a semiconductor substrate are thereafter covered with athermal oxide film for improving hot carrier resistance. When this stepis employed, however, side surfaces of the second layer made of metalsilicide such as tungsten silicide are more readily oxidized as comparedwith side surfaces of the first layer made of polysilicon, and henceoxide films abnormally grow on the side surfaces made of metal silicide.For example, Japanese Patent Laying-Open No. 7-183513 describes atechnique of preventing such abnormal growth of oxide films.

FIG. 10 is a sectional view of a semiconductor device having aconventional electrode structure described in the aforementionedgazette. Referring to FIG. 10, insulator films 101 for element isolationare formed on a silicon substrate 100 in the conventional semiconductordevice. Between the insulator films 101 for element isolation, apolysilicon film 103 is formed on the silicon substrate 100 through agate oxide film 102. A source region 110 and a drain region 111 areformed in the silicon substrate 100 on both sides of the polysiliconfilm 103. Rounded portions 108 and bird's beaks 109 are formed on theside walls of the polysilicon film 103 to be in contact with thepolysilicon film 103.

A natural oxide film 104 is formed on the polysilicon film 103. Atungsten silicide film 105 is formed on the natural oxide film 104. Asilicon nitride film 106 is formed on the tungsten silicide film 105.

Silicon nitride films 107 are formed to be in contact with the siliconnitride film 106, the tungsten silicide film 105 and the natural oxidefilm 104.

This electrode structure is fabricated as follows: First, the gate oxidefilm 102 is deposited on the silicon substrate 100, and the polysiliconfilm 103, the natural oxide film 104, the tungsten silicide film 105 andthe silicon nitride film 106 are deposited thereon in a layered manner.The silicon nitride film 106, the tungsten silicide film 105 and thenatural oxide film 104 are patterned into the shapes shown in FIG. 10.The silicon nitride films 107 are formed to cover the silicon nitridefilm 106, the tungsten silicide film 105 and the natural oxide film 104,and fully etched back into the shapes shown in FIG. 10. Thereafter thepolysilicon film 103 is etched into the shape shown in FIG. 10, andthereafter the side walls thereof are oxidized to form the roundedportions 108 and the bird's beaks 109.

According to this method, the side walls of the tungsten silicide film105, covered with the silicon nitride films 107 in the oxidation step,are inhibited from abnormal oxidation. On the other hand, the side wallsof the polysilicon film 103 covered with no nitride films are thermallyoxidized through the oxidation step. Consequently, the gate electrodecan be improved in reliability.

However, the conventional technique has the following problem: Thesilicon nitride films 107 are formed on the side walls of the tungstensilicide film 105. The width of the tungsten silicide film 105 isreduced due to the silicon nitride films 107. Therefore, the tungstensilicide film 105 is reduced in sectional area and increased in electricresistance. Thus, the reliability of the electrode structure isdeteriorated due to a signal delay or the like.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been proposed in order to solvethe aforementioned problem, and an object thereof is to provide areliable electrode structure.

The electrode structure according to the present invention includes afirst conductive layer having a first side wall and containing at leasteither polycrystalline silicon or amorphous silicon, a second conductivelayer, formed on the first conductive layer, having a second side walland containing a metal and silicon, and a side wall oxide film formed tobe in contact with the first side wall and the second side wall. Thefirst conductive layer and the second conductive layer contain nitrogenin the vicinity of the first and second side walls. The nitrogenconcentration in the second side wall is larger than the nitrogenconcentration in the first side wall.

In the electrode structure having the aforementioned structure, the sidewall oxide film is formed to be in contact with the first side wall andthe second side wall, whereby the first and second side walls can beprevented from crystal defects. The nitrogen concentration in the secondside wall is larger than the nitrogen concentration in the first sidewall, whereby larger quantities of nitrogen are added to the second sidewall containing the metal and silicon. Consequently, the growth rate ofthe side wall oxide film can be retarded on the second side wall, sothat the side wall oxide film can be prevented from abnormal growth onthe second side wall. Further, the side wall oxide film is formed on thesecond side wall so that no silicon nitride films are formed to bedirectly in contact with the second side wall, whereby a sufficientwidth can be ensured for the second conductive layer. Consequently, areliable electrode structure having small conductive resistance can beprovided.

Preferably, the nitrogen concentration in the second conductive layer isincreased toward the second side wall.

Preferably, the nitrogen concentration in the first side wall issubstantially identical to the nitrogen concentration in a centralportion of the first conductive layer.

Preferably, the first conductive layer is formed on a semiconductorsubstrate.

Preferably, the electrode structure further includes a gate oxide filmformed between the first conductive layer and the semiconductorsubstrate.

Preferably, the electrode structure further includes a surface oxidefilm formed on the semiconductor substrate continuing to the gate oxidefilm and the side wall oxide film.

Preferably, the first conductive layer has a portion reduced in widthtoward the semiconductor substrate. The electrode structure furthercomprises impurity regions formed on the semiconductor substrate on bothsides of the first conductive layer.

Preferably, the electrode structure further includes a silicon nitridefilm formed to cover the side wall oxide film.

A method of fabricating an electrode structure according to the presentinvention includes steps of successively stacking a first layercontaining at least either polycrystalline silicon or amorphous siliconand a second layer containing a metal and silicon, forming a secondconductive layer by etching the second layer, doping the side walls ofthe second conductive layer with nitrogen by exposing the secondconductive layer to an atmosphere containing nitrogen, forming a firstconductive layer by etching the first layer through a mask of the secondconductive layer having the side wall doped with nitrogen, and forming aside wall oxide film by oxidizing the side walls of the first and secondconductive layers.

In the method of fabricating an electrode structure having theaforementioned structure, the side wall of the second conductive layeris doped with nitrogen for thereafter forming the first conductive layerby etching the first layer through the mask of the second conductivelayer doped with nitrogen. Therefore, the side wall of the secondconductive layer is doped with large quantities of nitrogen, while theside wall of the first conductive layer is hardly doped with nitrogen.Consequently, the side wall of the second conductive layer can beinhibited from abnormal oxidation in the step of forming the side walloxide film by oxidizing the side walls of the first and secondconductive layers. Further, the side wall oxide film is formed on theside wall of the second conductive layer, whereby the width of thesecond conductive layer can be increased as compared with a case offorming a silicon nitride film to be in contact with the side wall ofthe second conductive layer. Consequently, the second conductive layercan be increased in sectional area and reduced in conductive resistance,for providing a reliable electrode structure.

Preferably, the step of successively stacking the first layer and thesecond layer includes a step of successively stacking the first layerand the second layer on a semiconductor substrate.

Preferably, the method of fabricating an electrode structure furtherincludes a step of implanting an impurity into the semiconductorsubstrate through masks of the first and second conductive layers afterforming the side wall oxide film.

Preferably, the method of fabricating an electrode structure furtherincludes steps of stacking a silicon oxide film and a silicon nitridefilm on the second layer and etching the silicon oxide film and thesilicon nitride film. The step of forming the second conductive layerincludes a step of etching the second layer through a mask of the etchedsilicon nitride film.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a semiconductor device having an electrodestructure according to a first embodiment of the present invention;

FIG. 2 is a graph showing distribution of nitrogen concentrations on aline II—II in FIG. 1;

FIG. 3 is a graph showing distribution of nitrogen concentrations on aline III—III in FIG. 1;

FIGS. 4 to 9 are sectional views showing first to sixth steps in amethod of fabricating the electrode structure shown in FIG. 1; and

FIG. 10 is a sectional view of a semiconductor device having aconventional electrode structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention is now described with referenceto the drawings.

(First Embodiment)

Referring to FIG. 1, an electrode structure according to a firstembodiment of the present invention includes a first conductive layer 11having side walls 11 s is as first side walls and containing at leasteither polycrystalline silicon or amorphous silicon, a second conductivelayer 12, formed on the first conductive layer 11, having side walls 12s as second side walls and containing a metal and silicon, and side walloxide films 15 formed to be in contact with the first and second sidewalls 11 s and 12 s. The first conductive layer 11 and the secondconductive layer 12 contain nitrogen in the vicinity of the side walls11 s and 12 s. The nitrogen concentration in the side walls 12 s islarger than the nitrogen concentration in the side walls 11 s.

The nitrogen concentration in the second conductive layer 12 isincreased toward the side walls 12 s. The nitrogen concentration in theside walls 11 s is substantially identical to the nitrogen concentrationin a central portion 11 c of the first conductive layer 11.

The first conductive layer 11 is formed on a silicon substrate 1employed as a semiconductor substrate.

The electrode structure further includes a gate oxide film 5 formedbetween the first conductive layer 11 and the silicon substrate 1. Thefirst conductive layer 11 has a portion reduced in width toward thesilicon substrate 1. The electrode structure further includes a sourceregion 3 and a drain region 4 formed on the silicon substrate 1 asimpurity regions on both sides of the first conductive layer 11. Theelectrode structure further includes a silicon nitride film 16 formed tocover the side wall oxide films 15.

A trench 1 h is formed in the silicon substrate 1, so that an isolationfilm 2 of silicon oxide is formed in the trench 1 h. The gate oxide film5 and a surface oxide film 6 are formed on a main surface 1 f of thesilicon substrate 1. The surface oxide film 6 is larger in thicknessthan the gate oxide film 5.

The first conductive layer 11 is formed on the gate oxide film 5. Thefirst conductive layer 11 contains at least either polycrystallinesilicon or amorphous silicon. The first conductive layer 11 may be madeof only polycrystalline silicon (polysilicon). Alternatively, the firstconductive layer 11 may be made of only amorphous silicon. Furtheralternatively, the first conductive layer 11 may contain both amorphoussilicon and polycrystalline silicon.

The first conductive layer 11 has the central portion 11 c and the sidewalls 11 s.

The second conductive layer 12 is formed to be in contact with the upperportion of the first conductive layer 11. The second conductive layer 12is formed by a central portion 12 c and the side walls 12 s. The secondconductive layer 12 contains a metal and silicon. The metal preferablyincludes at least one selected from a group consisting of tungsten,molybdenum, titanium, cobalt, tantalum and platinum. Therefore, thesecond conductive layer 12 is or may be made of tungsten silicide(WSi₂), molybdenum silicide (MoSi₂), titanium silicide (TiSi₂) cobaltsilicide (CoSi₂), tantalum silicide (TaSi₂) or platinum silicide(PtSi₂).

A silicon oxide film 13 is formed on the second conductive layer 12. Thesilicon oxide film 13 is substantially identical in width to the firstand second conductive layers 11 and 12.

A silicon nitride film 14 is formed on the silicon oxide film 13. Thesilicon nitride film 14 is also identical in width to the silicon oxidefilm 13.

The side wall oxide films 15 are formed to be directly in contact withthe side walls 11 s and 12 s. Further, the side wall oxide films 15 areformed continuing to the surface oxide film 6. The silicon nitride film16 is formed to cover the side wall oxide films 15.

A first conductive layer 11, a second conductive layer 12, a siliconoxide film 13, a silicon nitride film 14, side wall oxide films 15 and asilicon nitride film 16 similar to those formed on the main surface ifare formed also on the isolation film 2.

An interlayer isolation film 20 is formed on the silicon substrate 1 tocover the silicon nitride film 16. The interlayer isolation film 20consists of a silicon oxide film to which phosphorus or boron, forexample, is added. A contact hole 20 h is formed in the interlayerisolation film 20 to reach the drain region 4.

A plug layer 23 of polycrystalline silicon fills up the contact hole 20h. A lower electrode 24 of a capacitor is formed to be in contact withthe plug layer 23. A dielectric layer 21 is formed on the lowerelectrode 24, while an upper electrode 22 is formed on the dielectriclayer 21. The dielectric layer 21 can be formed not only by a siliconnitride film but also by a ferroelectric film of lead zirconate titanateor tantalum oxide. The lower electrode 24 can be cylindrically formedfor improving the capacitance of the capacitor.

Referring to FIG. 2, the nitrogen concentration in the second conductivelayer 12 reaches the maximum value a1 (1×10¹⁷ cm⁻³) on positions of x=0and x=L1 on a line II—II in FIG. 1, i.e., on the side walls 12 s. On theother hand, the nitrogen concentration reaches the minimum value a2(1×10¹³ cm⁻³) on the central portion 12 c.

In the first conductive layer 11, the nitrogen concentration in the sidewalls 11 s is substantially identical to the nitrogen concentration inthe central portion 11 s on a line III—III in FIG. 1. The nitrogenconcentration on the line III—III is substantially at a constant value b(1×10¹³ cm⁻³).

The width L1 of the first conductive layer 11 between the two side walls11 s is larger than the width L2 of the portion of the first conductivelayer 11 in contact with the gate oxide film 5. This is because bird'sbeaks 6 a are formed on the portion in contact with the gate oxide film5 to reduce the width of the first conductive layer 11.

A method of fabricating the semiconductor device shown in FIG. 1 is nowdescribed. Referring to FIG. 4, a resist film is applied onto the mainsurface if of the silicon substrate 1 and patterned by photolithography,for forming a resist pattern. The resist pattern is employed as a maskfor etching the silicon substrate 1, thereby forming the trench 1 h. Theisolation film 2 of silicon oxide is formed to fill up the trench 1 h.The main surface 1 f of the silicon substrate 1 is oxidized to form thegate oxide film 5. A polysilicon film 31, a tungsten silicide film 32, asilicon oxide film 33 and a silicon nitride film 34 are successivelystacked on the gate oxide film 5. A resist film is applied onto thesilicon nitride film 34 and patterned by photolithography, therebyforming a resist pattern 35.

Referring to FIG. 5, the silicon nitride film 34 and the silicon oxidefilm 33 are etched through the resist pattern 35 serving as a mask,thereby forming the silicon nitride film 14 and the silicon oxide film13.

Referring to FIG. 6, the resist pattern 35 is removed and thereafter thesilicon nitride film 14 is employed as a mask for etching the tungstensilicide film 32, thereby forming the second conductive layer 12. Then,RTA (rapid thermal anneal) processing is performed for maintaining thesecond conductive layer 12 in a nitrogen atmosphere for 30 to 60 secondsat a temperature of 850° C. to 1100° C. Thus, portions of the secondconductive layer 12 around the side walls 12 s are concentrically dopedwith nitrogen.

Referring to FIG. 7, the silicon nitride film 14, the silicon oxide film13 and the second conductive layer 12 are employed as masks for etchingthe polysilicon film 31, thereby forming the first conductive layer 11.At this time, the first conductive layer 11 has substantially equalwidths in the portions in contact with the gate oxide film 5 and thesecond conductive layer 12 respectively.

Referring to FIG. 8, the silicon substrate 1 is maintained in an oxygenatmosphere for 30 to 60 seconds at a temperature of 1000° C. to 1150°C., thereby forming the side wall oxide films 15. The side wall oxidefilms 15 are in contact with the side walls 11 s of the first conductivelayer 11, the side walls 12 s of the second conductive layer 12 and thesilicon oxide film 13. The surface oxide film 6 is formed on the mainsurface 1 f of the silicon substrate 1 following this oxidation. Asilicon oxide film readily grows on the main surface 1 f of the siliconsubstrate 1, not doped with nitrogen in the RTA step shown in FIG. 6. Onthe other hand, the side walls 12 s of the second conductive layer 12are doped with large quantities of nitrogen in the step shown in FIG. 6,and hence the side wall oxide films 15 can be inhibited from growing onthese portions. The thickness of the surface oxide film 6 is larger thanthat of the gate oxide film 5.

Referring to FIG. 9, the silicon nitride film 16 is formed to cover theside wall oxide films 15 and the surface oxide film 6. The interlayerisolation film 20 of silicon oxide is formed on the silicon nitride film16. A resist film is applied onto the interlayer isolation film 20 andpatterned by photolithography, thereby forming a resist pattern 40. Theresist pattern 40 is employed as a mask for etching the interlayerisolation film 20 and the silicon nitride film 16, thereby forming thecontact hole 20 h reaching the drain region 4 in a self-aligned manner.

Thereafter a polysilicon layer is formed to fill up the contact hole 20h. The polysilicon layer is fully etched back thereby forming the pluglayer 23. A polysilicon layer is formed on the plug layer 23 andpatterned into a prescribed shape, thereby forming the lower electrode24. The dielectric layer 21 and the upper electrode 22 are formed on thelower electrode 24 for completing the semiconductor device shown in FIG.1.

In the electrode structure having the aforementioned structure, the sidewalls 11 s and 12 s of the first and second conductive layers 11 and 12are first oxidized. Therefore, these portions can be prevented fromcrystal defects and improved in hot carrier resistance.

As shown in FIG. 2, the side walls 12 s of the second conductive layer12 consisting of tungsten silicide contain larger quantities of nitrogenas compared with the side walls 11 s of the first conductive layer 11,whereby the side wall oxide films 15 can be prevented from abnormalgrowth also when the side walls 12 s are oxidized in the subsequentstep. Therefore, a sufficient space can be ensured between the secondconductive layer 12 and the adjacent second conductive layer 12, so thatthe silicon nitride film 16 and the interlayer isolation film 20 canreliably fill up the space between the adjacent second conductive layers12.

Further, the side wall oxide films 15 are formed to be directly incontact with the side walls 11 s and 12 s of the first and secondconductive layers 11 and 12. Therefore, sufficient widths can be ensuredfor the first and second conductive layers 11 and 12, not to increasethe electric resistance of the first and second conductive layers 11 and12. Thus, a reliable electrode structure can be provided.

In addition, the thickness of the surface oxide film 6 is larger thanthat of the side wall oxide films 15, and the surface oxide film 6 hasthe bird's beaks 6 a. The bird's beaks 6 a are formed to cut into thefirst conductive layer 11, thereby increasing the distances between thesource and drain regions 3 and 4 and the first conductive layer 11.Thus, GIDL (gate induced drain leakage) resulting from an electric fieldcan be suppressed between the first conductive layer 11 and the drainregion 4. Therefore, deterioration of a pause refresh property can beprevented in the DRAM shown in FIG. 1.

The first embodiment described above can be modified in various ways.First, the first conductive layer 11 can be prepared by doping theaforementioned amorphous silicon or polysilicon with an impurity such asphosphorus or arsenic. The second conductive layer 12 may contain a highmelting point metal and silicon. The first conductive layer 11, thesecond conductive layer 12, the silicon oxide film 13 and the siliconnitride film 14 can be formed by CVD (chemical vapor deposition).

Further, the contact hole 20 h may be formed not by the self-alignedmethod shown in FIG. 9 but by a general method employing only the resistpattern 40 as a mask.

According to the present invention, a sufficient width can be ensuredfor the second conductive layer for providing a reliable electrodestructure.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. An electrode structure comprising: a firstconductive layer having a first side wall and containing at least eitherpolycrystalline silicon or amorphous silicon; a second conductive layer,formed on said first conductive layer, having a second side wall andcontaining a metal and silicon; and a side wall oxide film formed to bein contact with said first side wall and said second side wall, whereinsaid first conductive layer and said second conductive layer containnitrogen in the vicinity of said first and second side walls, and thenitrogen concentration in said second side wall is larger than thenitrogen concentration in said first side wall.
 2. The electrodestructure according to claim 1, wherein the nitrogen concentration insaid second conductive layer is increased toward said second side wall.3. The electrode structure according to claim 1, wherein the nitrogenconcentration in said first side wall is substantially identical to thenitrogen concentration in a central portion of said first conductivelayer.
 4. The electrode structure according to claim 1, wherein saidfirst conductive layer is formed on a semiconductor substrate.
 5. Theelectrode structure according to claim 4, further comprising a gateoxide film formed between said first conductive layer and saidsemiconductor substrate.
 6. The electrode structure according to claim5, further comprising a surface oxide film formed on said semiconductorsubstrate continuing to said gate oxide film and said side wall oxidefilm.
 7. The electrode structure according to claim 1, furthercomprising a silicon nitride film formed to cover said side wall oxidefilm.
 8. The electrode structure according to claim 1, furthercomprising a silicon oxide film overlying the second conductive layer.9. The electrode structure according to claim 8, wherein the width ofthe silicon oxide film is substantially the same as the width of thefirst and second conductive layers.
 10. The electrode structureaccording to claim 8, further comprising a first silicon nitride filmoverlying the silicon oxide film.
 11. The electrode structure accordingto claim 10, wherein the width of the first silicon nitride film issubstantially the same as the width of the first and second conductivelayers.
 12. The electrode structure according to claim 10, furthercomprising a second silicon nitride film overlying the first siliconnitride film and the side wall oxide film.
 13. An electrode structurecomprising: a first conductive layer formed on a semiconductor substratehaving a first side wall and containing at least either polycrystallinesilicon or amorphous silicon, wherein said first conductive layer has aportion reduced in width toward said semiconductor substrate; a secondconductive layer, formed on said first conductive layer, having a secondside wall and containing a metal and silicon; and a side wall oxide filmformed to be in contact with said first side wall and said second sidewall, wherein said first conductive layer and said second conductivelayer contain nitrogen in the vicinity of said first and second sidewalls, and the nitrogen concentration in said second side wall is largerthan the nitrogen concentration in said first side wall; and impurityregions formed on said semiconductor substrate on both sides of saidfirst conductive layer.